For whatever purpose furnaces are used, they have problems of deterioration due to the high temperatures involved and the chemical composition of the combustion products. Furnaces are customarily constructed of steel and have a combustion chamber lining of one or more layers of various kinds of brick, characterized broadly as firebrick. Such bricks are designed primarily to keep the steel walls from direct contact with the flame of the burner and the high heat of the combustion chamber itself. The better the shielding of the steel walls, the longer will be the life span of the basic structure, although the fire brick is replaced on a periodic basis, based on the temperature of the furnace, the useable hours and other factors.
Often there is a flue associated with the outlet from the furnace to receive the combustion products and convey them to an outside environment. In some instances the furnace has no flue and the combustion Products are discharged through a hole in the top of the furnace and products go directly into the air inside the building where the furnace is housed.
Often heat transfer equipment (commonly called recuperators), combustion product treatment apparatus or other equipment is incorporated into the flue where such is used. This equipment is usually placed as close to the furnace outlet as possible in order to increase heat recovery, to conserve space, to reduce the cost of high temperature ducting, etc. As a result, the downstream apparatus in the flue normally receives direct heat radiation from the interior of the furnace.
The heat radiation and the flow of the combustion products along the flue and past the apparatus incorporated into the flue can cause deterioration if care is not exercised. Normally the apparatus in the flue is designed in such a way that deterioration is acceptably slow when the system is operating normally. The flow of the combustion products through the apparatus becomes a means of carrying away the heat radiation. However, there are occasionally periods when the furnace is abruptly stopped, for example, when upsets occur or when production has to be stopped quickly because of emergencies elsewhere in the production line. In these instances, the burners are immediately shut off or turned down to low idle rate, and the flow of combustion products, out the flue and through the apparatus embodied in the flue, is stopped or decreased substantially. Since the furnace interior is still hot, however, heat radiation continues to pass into the flue and continues to heat the apparatus. Without the cooling effect of the flow of combustion products the temperature in the apparatus rises toward the temperature of the furnace. In cases where the heat transfer apparatus in the flue is not designed to handle these higher temperatures, the apparatus can be destroyed or permanently damaged very quickly.